Lowstand erosional seismic stratigraphy

ABSTRACT

A seismic interpretation method for identifying subsurface hydrocarbon bearing traps of Eocene/Paleocene age in valley fill depositional systems comprising as computer implemented modeling software and processed seismic data. The valley dispositional system is further defined by identifying field stratigraphy and erosional trapping mechanisms and confirming structural closure. The method further includes identifying structural aspects caused by sagging, rollover, and determining the presence of high amplitude events in the erosional trap.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 62/680,298 filed Jun. 4, 2018, the contents of which hereinincorporated by reference in their entirety.

BACKGROUND Field

Embodiments of This disclosure relates to techniques for the explorationof oil and gas. More specifically, it relates to lowstand erosionalseismic stratigraphy.

DESCRIPTION OF THE RELATED ART

The lowering of sea level is the primary mechanism associated with thedevelopment of incised valleys. In turn, incised valleys are theevidence for the existence of lowstand and transgressive deposition.

The erosion that creates many incised valleys is thought to be linked torelative sea-level fall, although climatically produced changes indischarge and/or sediment supply may independently cause incision, evenin areas far removed from the coast. In the case of valleys in coastalareas, fluvial deposition typically begins at the mouth of theincised-valley system when sea level is at its lowest point and expandsprogressively farther up the valley as the transgression proceeds,producing depositional onlap in the valley. Valley-fill sequences areknown to provide seals and reservoirs for stratigraphically trappedpetroleum accumulations.

Late Paleocene and early Eocene was a time of Thermal Maximum when theaverage global temperature was 8° C. warmer than today. The exact ageand duration of the event is uncertain but it is estimated to haveoccurred around 55.5 million years ago (between the Paleocene and Eocenegeological epochs).

The Thermal Maximum was a period of maximum sea level fluctuations.During sea level lows, incised valleys were created and filled with mudduring subsequent transgressions that separate intervals. These includeseveral Lower Wilcox intervals (Late Paleocene), which are majorhydrocarbon bearing formations along the U.S. Gulf Coast.

Galloway et al (2011) taught that sequence stratigraphy is a methodologythat provides a framework for the elements of any depositional setting,facilitating paleogeographic reconstructions and the prediction offacies and lithologies away from control points. This framework tieschanges in stratal stacking patterns to the responses to varyingaccommodation and sediment supply through time. Stratal stackingpatterns enable determination of the order in which strata were laiddown and explain the geometric relationships and the architecture ofsedimentary strata. The sequence stratigraphic framework also providesthe context within which to interpret the evolution of depositionalsystems through space and time. The main tool used in sequencestratigraphic analysis is the stacking pattern of strata and the keystratigraphic boundary surfaces are defined by different stratalstacking patterns. The definition of these units is independent oftemporal and spatial scales, and of the mechanism of formation.

The sequence stratigraphic approach relies on the observation of stratalstacking patterns and the key stratigraphic boundary surfaces aredefined by different stacking patterns. Construction of this frameworkensures the success of the method in terms of its objectives to providea process-based understanding of the stratigraphic architecture.

The surfaces that are selected as sequence boundaries vary from onesequence stratigraphic approach to another. In practice, the selectionis typically a function of which surfaces are best expressed within thecontext of each situation, depending upon tectonic setting, depositionalsetting, types of available data and the scale of observation. The highdegree of variability in the expression of stratigraphic boundarysurfaces requires the adoption of a methodology that is sufficientlyflexible to accommodate the wide range of possible scenarios.

Devine and Wheeler (1989) taught that exploration potential exists whereupdip convex curvature of mudstone filled valleys or the intersection ofprimary and secondary drainages form lateral barriers to petroleummigration. These barriers can form stratigraphic traps or enhancestructural closure. Eocene-Paleocene Epoch (EPE) strata constitute aregressive clastic wedge of fluvial and deltaic deposits formed during agradual sea-level rise on passive margins. Multiple episodes ofsea-level lowstand occurred during the rise; each produced deepincisement of river valleys across the Tertiary coastal plain and shelfareas. The relative age of individual valleys can be determined based onthe ordered occurrence of valley-fill tops within a series oftime-stratigraphic markers correlated in the intervening and surroundingEPE.

Most EPE strata include valley-fills that represent drowning of riverineembayment's in shoreline settings during transgression. Thesevalley-fill sequences consist dominantly of mudstones with minorsandstones, providing limited reservoir potential. Exploration potentialcan exist where thick mudstone deposits can serve as lateral andvertical seals for petroleum migration. Potential reservoirs exist inEPE fluvial and deltaic strata that have been truncated below theunconformity at the base of the valley sequence.

Stacking of multiple-aged valleys is common in EPE and has producedthick sequences of relatively impermeable valley-fill mudstone.Stratigraphic traps can be anticipated along major updip convexcurvatures of the valley trends or at the intersections of primary andsecondary drainages. These situations can also enhance structuralclosures. Accurate mapping of the valley margin trends by thetime-stratigraphic methods is crucial for detailed delineation oftrapping geometries.

SUMMARY

One embodiment of this disclosure comprises a stratigraphic trappingsystem created by meander bends and structural noses forming trapsagainst a shale filled erosional sequence.

One embodiment of this disclosure comprises a stratigraphic trappingsystem created by erosional channels/canyons in the lower (older)erosional sequence, which occurred during a regressive cycle. Thesechannel/canyons became shale filled during the next transgressive cyclecreating traps.

One embodiment of this disclosure comprises a stratigraphic trappingsystem created by erosional channels/canyons in the middle (younger)erosional sequence, which occurred during a regressive cycle. Theseeroded into the previous transgressive cycle of fluvial—deltaicdeposition, forming erosional remnants. These channel/canyons becameshale filled during the next transgressive cycle creating traps.

One embodiment of this disclosure comprises a stratigraphic trappingsystem created by submarine canyon and erosional gully lowstand sanddeposits being preserved as erosional remnants between the lowererosional sequence and the middle erosional sequence. These sanddeposits are completely encased in shale between the lower and middleerosional sequences.

One embodiment of this disclosure comprises picking the base of thelower erosional sequence (BLES) boundary using a 3-D seismic sectionrotated parallel to the incised valley.

One embodiment of this disclosure comprises picking the top of the lowererosional sequence (TLES) boundary using a 3-D seismic section rotatedperpendicular to paleo-coast line.

One embodiment of this disclosure comprises picking the base of themiddle erosional sequence (BMES) boundary using a 3-D seismic sectionrotated perpendicular to paleo-coast line.

One embodiment of this disclosure comprises determining if noticeablesag occurs in the underlying BLES indicative of an overlying productiveinterval.

One embodiment of this disclosure comprises determining arching effectat the BMES due to differential compaction indicative of an underlyingproductive interval.

One embodiment of this disclosure comprises determining the presence ofa high amplitude event associated with hydrocarbon productive intervals.

One embodiment of this disclosure comprises optimizing the color bar toenhance interpreter's ability to pick key boundaries.

BRIEF DESCRIPTION OF THE DRAWING

So that the manner in which the above recited features can be understoodin detail, a more particular description of the embodiments brieflysummarized above may be had by reference to the embodiment below, someof which are illustrated in the appended drawing. It is to be noted,however, that the appended drawing illustrates only typical embodimentsand are therefore not to be considered limiting of its scope, for theembodiments may admit to other equally effective embodiments.

FIG. 1 shows a eustatic curve during the Eocene Paleocene Epochs

FIG. 2 shows a block diagram of an EPE depositional model

FIG. 3 shows a section view schematic of an erosional truncation trap.

FIG. 4 shows a map view schematic of an erosional truncation trap

FIG. 5 shows a section view schematic of a basal erosional remnant trap

FIG. 6 shows a map view schematic of a basal erosional remnant trap

FIG. 7 shows a section view schematic of an intermediary erosionalremnant trap

FIG. 8 shows a map view schematic of an intermediary erosional remnanttrap

FIG. 9 shows a section view schematic of an inter-channel erosionalremnant trap

FIG. 10 shows a map view schematic of an inter-channel erosional remnanttrap

FIG. 11 shows interpreted 3-D seismic section for analog field with abasal erosional remnant trap

FIG. 12 shows interpreted 3-D seismic section for analog fieldinter-channel erosional remnant trap

DETAILED DESCRIPTION

Lowstand Erosional Seismic Stratigraphy (LESS) interpretation method isused to identify hydrocarbon bearing traps within incised valley systemsspecifically in the late Paleocene Era. The incised valleys form becausethe transport capacity of a river exceeds its sediment supply. Anincised-valley system is defined as a fluvially eroded, elongatetopographic low that is characteristically larger than a single channeland is marked by an abrupt seaward shift of depositional facies across aregionally mappable sequence boundary at its base.

With sea level rise due to global warming, rivers erode the land massand deposit fluvial/deltaic sands in shallow water.

With a decline in sea level, the rivers make their way towards the newshoreline. The previously deposited fluvial/deltaic sands are nowexposed and subject to erosion. These sands are transported anddeposited as lowstand deposits along the newly established shoreline.

With another rise of sea level. The sea transgresses over the land mass.The entrenched shoreline valleys and submarine canyons which formedduring the previous sea level drop are back-filled with primarilyestuarine muds that later compact into shale. As the sea level continuedto rise, fluvial/deltaic sands were once again deposited into theshallow water near shore over the top of the previously eroded nowshale-filled section.

With another decline in sea level, the rivers make their way towards thenew shoreline. The previously deposited fluvial/deltaic sands are nowexposed and subject to erosion. Once again, these sands are transportedand deposited offshore along the newly established shoreline.

As sea level rises again, the incised valleys once more are back-filledwith the near shore muds until the water depth is deep enough for thedeposition of clean deltaic sands by the river systems.

FIG. 1 is a diagram showing eustatic curves for the late Paleocene andEocene Epochs. Column 10 entitled Group refers to the local or regionalnames given to groups of subsurface formations. Column 20 entitledEustatic Curves shows the global change in sea level shown in meters andthe duration of the change shown in millions of years. Column 30 is therelative geologic age corresponding to the eustatic curves.

FIG. 2 is geological model built on a platform 10 with platform margins20 that display key depositional features. Incised valleys 25 arecomprised of bay 30 in high stand 45 and drowned valley 40 emanatingfrom shore line 50 into the first lowstand 60. Potential hydrocarbonbearing traps are deposited in lowstand wedges 100 at the outflow ofincised valleys 25, submarine canyon 130 and gullies 90 and into thesecond lowstand 70. In addition, slumps 120 may also occur in thesubmarine canyon.

FIG. 3 is a section view of an erosional truncation trap model 10 shalefilled with middle erosional sequence 20 that terminates againstunconformity 30. The erosional truncation trap was created by meanderbends and structural noses of beds of sand 50 and shale 40 forming astraps against the shale filled erosional sequence 20. Hydrocarbon 60 canbe potentially trapped in sand 50.

FIG. 4 is a map view of and erosional truncation trap model 10 shalefilled with middle erosional sequence 20 that terminates againstunconformity 30. The erosional truncation trap was created by meanderbends and structural noses depicted by contours 70 of beds of sand 50forming as traps against the shale filled erosional sequence 20.Hydrocarbon 60 can be potentially trapped in sand 50.

FIG. 5 is a section view of a basal erosional remnant trap model 10shale filled with Lower Erosional Shale Filled sequence 20 thatterminates against unconformity 30. The basal erosional remnant trap wascreated by channels/canyons in the lower (older) erosional sequence,which occurred during an earlier regressive cycle. Thesechannels/canyons eroded into the previous transgressive cycle offluvial—deltaic deposition depositing shales 40 and sands 50 and laterforming erosional remnants that became shale filled during the nexttransgressive cycle. Hydrocarbons 60 were introduced into the basalerosional remnant trap 10.

FIG. 6 is a map view of a basal erosional remnant trap model 10 shalefilled with Lower Erosional Shale Filled sequence 20 that terminatesagainst unconformity 30. The basal erosional remnant trap was created bychannels/canyons in the lower (older) erosional sequence, which occurredduring an earlier regressive cycle. These channels/canyons eroded intothe previous transgressive cycle of fluvial—deltaic depositiondepositing sands 50 and later forming erosional remnants that becameshale filled during the next transgressive cycle. Hydrocarbons 60 wereintroduced into the basal erosional remnant trap 10. Structural contoursare shown by 70.

FIG. 7 is a section view of an intermediary erosional remnant trap model10 shale filled with middle erosional shale-filled sequence 20 thatterminates against unconformity 30. This trap was created by erosionalchannels/canyons in the middle (younger) erosional sequence, whichoccurred during a previous regressive cycle and sit above the LowerErosional Shale Filled Sequence 70. These channels/canyons eroded intothe previous transgressive cycle of fluvial—deltaic depositiondepositing shales 40 and sands 50 and later forming erosional remnantsthat became shale filled during the next transgressive cycle.Hydrocarbons 60 were introduced into the intermediary erosional remnanttrap 10.

FIG. 8 is a map view of an intermediary erosional remnant trap model 10shale filled with middle erosional shale-filled sequence 20 thatterminates against unconformity 30. This trap was created by erosionalchannels/canyons in the middle (younger) erosional sequence, whichoccurred during a previous regressive cycle and sit above the lowererosional shale filled sequence 20. These channels/canyons eroded intothe previous transgressive cycle of fluvial—deltaic depositiondepositing sands 50 and later forming erosional remnants that becameshale filled during the next transgressive cycle. Hydrocarbons 60 wereintroduced into intermediary erosional remnant trap 10. Structuralcontours are shown by 70.

FIG. 9 is a section view of an inter-channel erosional remnant trapmodel 10 shale filled with middle erosional shale filled sequence 20that terminates against unconformity 30. inter-channel erosional remnanttrap was created by submarine canyon and erosional gully lowstand sanddeposits 60 being preserved as erosional remnants between the lowererosional sequence and the middle erosional sequence. These sanddeposits 60 are completely encased in shale between the lower 70 andmiddle 20 erosional sequences. The sands 60 are of high reservoirquality and are usually gas/condensate productive.

FIG. 10 is a map view of an inter-channel erosional remnant trap model10 shale filled with middle and lower erosional shale filled sequence 20that terminates against unconformity 30. Inter-channel erosional remnanttrap was created by submarine canyon and erosional gully lowstand sanddeposits 60 being preserved as erosional remnants between the lowererosional sequence and the lower middle erosional sequence. These sanddeposits 60 are completely encased in shale between the lower 70 andlower middle 20 erosional sequences. The sands 60 are of high reservoirquality and are usually gas/condensate productive.

FIG. 11 is an optimized color bar 3-D seismic section 10 displaying anactual analog oilfield 20 with inter-channel erosional remnant trap.Analog field 20 is bounded by base lower erosional sequence 30. Keyhydrocarbon indicators include arching effect 70 at the base middleerosional sequence 50 due to differential compaction and brightening ofhigh amplitude event 50 in analog field 20. Also shown are in-planeprojected wellbore 60 and out-of-section projected wellbores 70.

FIG. 12 is an optimized color bar 3-D seismic section 10 displaying anactual analog oilfield 20 with an inter-channel erosional remnant trap.Analog field 20 is bounded by base lower erosional sequence 30, toplower erosional sequence 40 and base middle

1. A seismic interpretation method for identifying subsurfacehydrocarbon bearing traps of Eocene/Paleocene age in valley filldepositional systems comprising: a computer implemented modelingsoftware with processed or reprocessed seismic data, the representationbeing displayed on a graphic user interface; identifying and correlatingstratigraphic boundary surfaces using analogous field data; adjustingcomputer implemented modeling software color bar to optimizeinterpretability of previously recorded seismic traces thus enablinginterpretation beyond the analogous field data; identifying fieldstratigraphy, erosional trapping mechanism and confirming structuralclosure; identifying structural aspect caused by differentialcompaction; identifying structural aspect caused by sagging androllover; determining the presence of high amplitude events in theerosional trap.
 2. The seismic interpretation method of claim 1, whereinthe valley fill depositional system is referred to as lowstand erosionalseismic stratigraphy (LESS).
 3. The computer implemented method of claim1, wherein the representation is displayed upon the graphic userinterface using a 2D, 3D, or 4D arrangement.
 4. The erosional trappingmechanism of claim 1, wherein the trap is an erosional truncation trap.5. The erosional trapping mechanism of claim 1, wherein the trap is abasal erosional remnant trap.
 6. The erosional trapping mechanism ofclaim 1, wherein the trap is an intermediary erosional remnant trap. 7.The erosional trapping mechanism of claim 1, wherein the trap is aninter-channel erosional remnant trap.
 8. The erosional trappingmechanism of claim 1, wherein the stratigraphic trapping system iscreated by meander bends and structural noses forming traps againstshale filled erosional sequences.
 9. The erosional trapping mechanism ofclaim 1, wherein the stratigraphic trapping system is created byerosional channels/canyons in the lower (older) erosional sequence,which occurred during a regressive cycle and later became shale filledduring the next transgressive cycle thus creating traps.
 10. Theerosional trapping mechanism of claim 1, wherein the stratigraphictrapping system is created by erosional channels/canyons in the middle(younger) erosional sequence which occurred during a regressive cycle.11. The erosional sequence of claim 10, wherein the previoustransgressive cycle of fluvial-deltaic deposition formed erosionalremnants which became shale filled during the next regressive cycle thuscreating traps.
 12. The erosional trapping mechanism of claim 1, whereinthe stratigraphic trapping system is created by submarine canyon anderosional gully lowstand sand deposits being preserved as erosionalremnants between the lower erosional sequence and the middle erosionalsequence.
 13. The erosional trapping mechanism of claim 12, wherein thesand deposits are completely encased in shale between the lower andmiddle erosional sequences.
 14. The stratigraphic boundary surfaces ofclaim 1, wherein the lower most surface is the lower erosional sequenceboundary (BLES).
 15. The lower erosional sequence boundary of claim 14,wherein TLES is the top of the lower erosional sequence boundary. 16.The stratigraphic boundary surfaces of claim 1, wherein the middlesurface is the middle erosional sequence boundary (BMES).
 17. Thestructural aspect caused by differential compaction of claim 1, whereinif noticeable arching at the middle erosional sequence boundary (BMES),then indicative of an overlying hydrocarbon productive interval.
 18. Thestructural aspect caused by sagging of claim 1, wherein the noticeablesag in the underlying lower erosional sequence boundary BLES isindicative of an overlying hydrocarbon productive interval.
 19. The highamplitude events of claim 1, wherein is associated with a hydrocarbonproductive interval.
 20. The analogous field data of claim 1, whereinthe data is comprised of electric logs, log cross sections and coredata.